1. Field of the Invention
Embodiments of the present invention relate generally to methods and devices for depositing viscous materials onto a printed wiring board. In one aspect, the present invention relates to methods and devices for compressing viscous materials, such as solder paste, through openings in a perforated substrate, such as a patterned screen or stencil.
2. Description of Related Art
Surface Mount Technology (SMT) involves placing circuit components onto circuit paths embedded on the upper surface of a printed wiring board and then soldering the components in place by a process called xe2x80x9creflow solderingxe2x80x9d. Before the circuit component is placed on the printed wiring board, however, it is desirable to apply solder paste to the area on the printed wiring board where the component is to be soldered into place.
Conventional methods do exist to deposit (xe2x80x9cprintxe2x80x9d) solder paste onto desired areas of a printed wiring board by forcing the paste through openings in a substrate (e.g., a stencil) placed in intimate contact with the printed wiring board.
U.S. Pat. No. 4,622,239 describes such a method and device for dispensing viscous materials. The method includes forcing a viscous material from a housing through an opening and depositing it onto a stencil between a pair of flexible members (parallel squeegee blades) which depend from the housing on either side of the opening and are in contact with the stencil. The ends of the flexible members are not connected and remain open ended. The viscous material, accordingly, is not contained within an enclosed area when it is deposited on the surface of the stencil. Movement of the housing and the flexible members horizontally across the stencil causes the trailing flexible member to force the viscous material through the openings in the stencil. U.S. Pat. No. 4,720,402 describes a similar method and device except that the leading flexible member is raised off of the stencil during movement of the housing.
U.S. Pat. Nos. 5,133,120 and 5,191,709 describe methods for filling through-holes of a printed wiring board via a mask with pressurized conductive filler material by means of a nozzle assembly unit having a nozzle tip member. The nozzle tip member, however, is designed only to dispense the pressurized conductive filler material through the mask to a single through-hole. The nozzle tip member then xe2x80x9cscansxe2x80x9d the printed wiring board for a second through-hole to fill. The nozzle tip member has a blunt end section which rests on the mask and a circular exit, the diameter of which may be increased or decreased by changing the nozzle tip member. The nozzle tip member dispenses the filler material without controlling unwanted flow of xe2x80x9cexcessivexe2x80x9d filler material back through the stencil. Additionally, the nozzle tip member does not define a contained environment where xe2x80x9ccompressionxe2x80x9d of the filler material takes place through the mask followed by the immediate shearing off of the filler material within that contained environment from the surface of the stencil. In fact, the nozzle tip member itself provides no effective means for shearing off filler material from the top of the stencil, rather, after the through hole is filled and filler material xe2x80x9cbacks upxe2x80x9d through the stencil, the nozzle tip member moves forward whereupon the xe2x80x9cexcessivexe2x80x9d filler material is then wiped off by a separate, single, flexible squeegee member which is designed for unidirectional use only.
Unfortunately, these conventional efforts do not provide a contained environment for compression of viscous material through holes in a stencil and shearing of viscous material within the contained environment from the upper surface of the stencil. Reliance upon squeegee movement to force the viscous material, such as solder paste, through the stencil openings can lead to damage and eventual failure of both the squeegee blades and the stencil due to repeated friction. Since conventional efforts do not provide a contained environment in which compression and shearing is accomplished, waste of the viscous material is frequently encountered.
Conventional efforts, therefore, (1) fail to maximize the efficiency of printing solder paste onto a desired area of a printed wiring board and (2) fail to minimize waste of the solder paste during the printing process. A need therefore exists to develop a method for printing solder paste onto a printed wiring board and a device suitable for use therewith which overcomes the deficiencies of the conventional efforts.
Moreover, these conventional methodologies and assemblies do not substantially ensure that the viscous material is dispensed and selectively placed upon the various portions of the circuit board, through the perforated stencil, at a substantially equal velocity. These conventional methodologies and assemblies also do not substantially ensure that the xe2x80x9cbackpressurexe2x80x9d formed within the compression head and/or within or through those portions of the exit aperture overlaying solid or xe2x80x9cnon-perforatedxe2x80x9d portions of the stencil and/or overlaying those perforated stencil portions which are filled with paste or viscous material, is substantially identical or uniform. Hence, these conventional methodologies and assemblies provide an undesirable and xe2x80x9cnon-uniformxe2x80x9d velocity and pressure or xe2x80x9cbackpressurexe2x80x9d distribution or xe2x80x9cprofilexe2x80x9d. Particularly, a velocity xe2x80x9cprofilexe2x80x9d is the numerical value of the velocity of the emitted viscous material at various locations within the exit aperture which overlay perforated portions of the stencil. A pressure xe2x80x9cprofilexe2x80x9d is the numerical value of the pressure or xe2x80x9cbackpressurexe2x80x9d created through and/or within the various locations of the exit aperture by the exiting viscous material which encounters xe2x80x9csolidxe2x80x9d stencil portions (e.g., paste filled openings or portions of the stencil having no perforations).
Particularly, the viscous material or solder xe2x80x9cpastexe2x80x9d is typically and selectively received within a top portion of a compression head through a material reception aperture. The received viscous material is made to travel through the compression head before exiting through an exit aperture or a slotted opening which is usually formed within the bottom portion of the compression head.
Most of the received viscous material traverses through the compression head along a path which is substantially aligned with the reception aperture. This path, in the above-described arrangement, typically lies along the center of the compression head and, more particularly, typically lies between the viscous material reception aperture (e.g., the location where the viscous material enters the head) and the exit opening or exit aperture. The viscous material traveling along this aperture-aligned path selectively emanates from or is selectively emitted from the compression head at a substantially higher velocity than the material which traverses along the various ends or outer wall portions of the compression head, and forms greater amounts of pressure or xe2x80x9cbackpressurexe2x80x9d than the viscous material traveling along these other paths. This non-uniform velocity and pressure distribution or profile causes uneven amounts of the viscous material to be deposited upon the stencil, thereby causing much of the deposited paste or viscous material to be undesirably wasted and concomitantly reducing the overall quality of the printed and created circuit board.
To provide improved pressure and velocity profile uniformity of the viscous material, a series of perforated plates or islands are oftentimes deployed and used within the compression head. These objects are somewhat effective to redistribute the flow of the received viscous material in a manner which causes more of the received viscous material to flow along the end or wall portions of the compression head and to increase the flow resistance along the previously described aperture-aligned path of and/or within the compression head. While these plates or islands do reduce some of the foregoing non-uniformity, they suffer from some drawbacks.
By way of example and without limitation, the use of these plates and/or islands requires that a relatively costly and time-consuming modification of the compression head be made (e.g. the compression head must be xe2x80x9copenedxe2x80x9d and these objects must be securely fitted or positioned within the head). These plates or islands further cause relatively large pressure losses to occur within the compression head, thereby requiring the viscous material to be communicated to the compression head at a relatively high pressure. This relatively xe2x80x9chigh pressure injectionxe2x80x9d arrangement causes the creation of significant amounts of xe2x80x9cback pressurexe2x80x9d within the compression head. Particularly, this undesirably created xe2x80x9clarge amount of back pressurexe2x80x9d frequently causes the wiper blades to undesirably xe2x80x9clift upxe2x80x9d or become disengaged from the surface of the stencil, thereby preventing the blades from properly wiping the stencil of the selectively deposited viscous material and causing circuit boards of an unacceptably poor quality to be created. Further, this required high pressure, in combination with certain xe2x80x9cstagnation regionsxe2x80x9d within the compression head which are generally located between the various perforations or openings in the diffuser plates or islands, causes compaction of the received viscous material and deposition onto the diffusion plates or islands, thereby undesirably xe2x80x9ccloggingxe2x80x9d the flow path and requiring frequent and time consuming maintenance and cleaning of the deployed plates/islands.
There is therefore a need to provide a viscous material compression head which overcomes the various and previously delineated drawbacks associated with prior techniques and assemblies, and which selectively receives and which selectively emits viscous material along and through an exit opening or aperture, the selectively emitted viscous material having a substantially uniform velocity profile, and the compression head assembly having a substantially uniform pressure profile. Particularly, the term xe2x80x9csubstantially uniform velocity and pressure profilexe2x80x9d means that the velocity of the exiting material at each xe2x80x9cpointxe2x80x9d or location of and/or within the viscous material dispensation or exit aperture which overlays a perforated portion of the stencil is substantially equal or similar. That is, the emitted viscous material passes through the exit aperture and enters the stencil perforations at a substantially uniform velocity. Further, the amount of xe2x80x9cbackpressurexe2x80x9d created at various locations within the aperture which overlay xe2x80x9csolidxe2x80x9d or non-perforated portions of the stencil (e.g., such as xe2x80x9cpaste filled openingsxe2x80x9d) is also respectively and substantially equal at each such xe2x80x9cpointxe2x80x9d or location within the exit aperture.
The present invention includes a novel apparatus and method for dispensing viscous material through openings in a stencil. Embodiments of the present invention include a process herein referred to as xe2x80x9ccompression printingxe2x80x9d wherein pressure is applied to a viscous material within a contained environment defined by a compression head cap so as to compress it through openings in a stencil.
The apparatus of the present invention includes a reservoir containing viscous material which is operably connected to a pressure source. The reservoir is in fluid communication with a housing which terminates in a substantially uniform opening defined by a compression head cap formed from contiguous walls. During operation of the apparatus, the compression head cap is placed in contact with a stencil having a plurality of openings therein. The compression head cap and the stencil form a contained environment. The pressure source then applies pressure against the viscous material contained in the reservoir forcing it from the reservoir into the housing and to the compression head cap. The contiguous walls of the compression head cap act to contain and to direct flow of the pressurized viscous material to the top surface of the stencil and then through the openings in the stencil.
It is accordingly an object of the present invention to provide a novel apparatus for compressing a viscous material through openings in a stencil by means of a pressure source. It is a further object of the present invention to increase the efficiency of printing viscous material onto a desired area of a printed wiring board and to minimize waste of the viscous material during the printing process.
According to another aspect of the present invention a compression head is provided which selectively receives and which selectively emits viscous material, and which causes the selectively emitted viscous material to have a substantially uniform velocity profile. Particularly, the compression head includes a chamber having a certain shape. The chamber selectively receives the viscous material and selectively emits the received viscous material. The certain shape is effective to cause the dispensed viscous material to have a substantially uniform velocity profile and to cause a substantially uniform pressure profile to be created.
According to another aspect of the present invention a method is provided to receive and selectively dispense viscous material at a substantially uniform velocity. The method includes the steps of providing a head which selectively receives the material at a certain location within the head and which includes a material dispensation aperture which allows the received material to be selectively dispensed as the received material flows through the head. The method further includes the step of narrowing a portion of the provided head, effective to cause the material to be selectively dispensed at a substantially uniform velocity along the material dispensation aperture.
Other objects, features or advantages of the present invention will become apparent -from the following description taken in conjunction with the accompanying drawings.